These instructions should work and have been tested on macOS Catalina 10.15.4 and Kindle.app 1.28.

1. Install Calibre. Calibre will be used to view the de-DRMed ebook and convert to other formats.
2. Install the DeDRM Calibre plugin. This will be used to actually break the DRM on the ebooks.
3. Install Kindle.app.
4. Run sudo chmod -x /Applications/Kindle.app/Contents/MacOS/renderer-test in the terminal. This forces Kindle.app to download ebooks in an older format, which is necessary for the DeDRM plugin to work.
5. Download an ebook via Kindle.app.
6. Locate the .azw file for the ebook inside of ~/Library/Containers/com.amazon.Kindle/Data/Library/Application\ Support/Kindle/My\ Kindle\ Content.
7. Import this ebook file into Calibre. The ebook should now be viewable and convertable.